= Mass-specific myocardial oxygen delivery
\b[Mass-specific myocardial oxygen delivery] expresses oxygen inflow relative to the mass of the represented myocardial tissue. If total <coronary oxygen delivery> is $D_{\mathrm O_2}$ to mass $m$, the per-mass quantity is $D_{\mathrm O_2}/m$. Reporting per $100\,\mathrm g$ gives
$$
D_{100}=\frac{100\,\mathrm g}{m}D_{\mathrm O_2}.
$$
The mass and inflow must refer to compatible territories. Normalizing a branch flow by an unrelated whole-heart mass changes the quantity rather than simply changing its units.
An original example has two regions: $50\,\mathrm g$ receiving $15\,\mathrm{mL\ O_2\,min^{-1}}$, and $150\,\mathrm g$ also receiving $15$. Their mass-specific deliveries are $30$ and $10\,\mathrm{mL\ O_2\,min^{-1}}$ per $100\,\mathrm g$. The combined value is
$$
D_{100,\rm combined}=\frac{100}{50+150}(15+15)=15.
$$
It is the mass-weighted mean $0.25(30)+0.75(10)$, not the unweighted mean $20$. Equal total inflows do not imply equal delivery per tissue mass.
Normalization does not remove measurement uncertainty. For small independent input errors,
$$
\frac{u_{D/m}^2}{(D/m)^2}\simeq\frac{u_D^2}{D^2}+\frac{u_m^2}{m^2}.
$$
Relative uncertainties $5\%$ in delivery and $10\%$ in mass combine to about $11.2\%$. With correlated errors, the relative covariance term $-2\operatorname{Cov}(D,m)/(Dm)$ must also be included.<Evaluation of measurement data — Guide to the expression of uncertainty in measurement by JCGM>[Sections 5.1–5.2]{ref}
An experimental flow measurement still needs separate oxygen-content and mass information. The primary coronary thermodilution apparatus estimated volumetric flow in a selected artery using infusion and temperature sensing.<Direct volumetric blood flow measurement in coronary arteries by thermodilution by Wilbert Aarnoudse and others>[Abstract, Methods]{ref} A mass-normalized oxygen-delivery estimate therefore combines several observations and a territory model.
Finally, a regional mean can conceal spatial variation. Two regions can share the same average per-mass delivery while distributing flow differently within them. The denominator makes comparisons explicit; it does not establish uniform perfusion or equal <coronary oxygen uptake>.
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